add torch.jit.is_scripting api (#25955)
Summary:
The PR https://github.com/pytorch/pytorch/pull/25263 was based on got reverted and ghimport got confused. Relanding here.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25955
Differential Revision: D17296727
Pulled By: eellison
fbshipit-source-id: 96200d3ef4c86f0d9907dc41b05619cb33bf2bab
